Belt separator systems (BSS) are used to separate the constituents of particle mixtures based on the charging of the different constituents by surface contact (i.e., the triboelectric effect). FIG. 1 shows a belt separator system 10, such as disclosed in U.S. Pat. Nos. 4,839,032 and 4,874,507, which are hereby incorporated by reference in their entirety. Belt separator system 10 includes parallel, spaced electrodes 12 and 14/16 arranged in a longitudinal direction defined by longitudinal centerline 25 and belt 18 traveling in the longitudinal direction between the spaced electrodes. The belt forms a continuous loop which is driven by a pair of end rollers 11, 13. A particle mixture is loaded onto belt 18 at feed area 26, between electrodes 14 and 16. Belt 18 includes counter-current traveling belt segments 17 and 19 moving in opposite directions for transporting the constituents of the particle mixture along the lengths of the electrodes 12 and 14/16.
An electric field is created in a transverse direction between electrodes 12 and 14/16 by applying a potential to electrode 12 of polarity opposite to a potential applied to electrodes 14/16, e.g., electrode 12 has a positive potential, and electrodes 14/16 have a negative potential. As the constituents of the particle mixture are transported along the electrodes by belt 18, the particles become charged and experience a force in a direction transverse to longitudinal centerline 25 of system 10, due to the electric field. When electrode 12 is positively charged and electrodes 14/16 are negatively charged, the electric field moves the positively charged particles toward electrodes 14/16 while the negatively charged particles move toward electrode 12. Ultimately, each particle is transferred toward one of product removal section 24, and reject removal section 22, depending upon the sign of charge of the particular particle as well as the sign of charge of the electrodes.
The charge that a particle develops determines the polarity of the electrode to which it will be attracted, and, therefore, the direction in which belt 18 will carry the particle. This charge is determined by the relative electron affinity of the material--a function of the energy needed to remove an electron from the surface of the particle (i.e., the work function of the particle). When two particles contact, the particle with the higher work function gains electrons and becomes negatively charged, while the particle with the lower work function loses electrons and becomes positively charged. For example, mineral oxide particles have relatively high work functions, and coal species have relatively low work functions; thus, during separation of these two particles in system 10, the coal becomes positively charged while the mineral oxide becomes negatively charged.
Typically, when separating mineral oxide particles from coal, system 10 is arranged such that belt 18 moves in a counter-clockwise direction as shown in FIG. 1. Electrodes 14/16 (adjacent belt segment 19) are at negative potential, and electrode 12 (adjacent belt segment 17) is at positive potential. With this arrangement, the positively-charged coal particles are moved to the product removal section 24 by belt section 19, while the negatively-charged mineral oxide particles are moved to the reject removal section 22 by belt section 17.
It is possible to operate belt system 10 in three other modes by varying the travel direction of the belt and/or the polarity of the electrodes. In a second mode of operation, belt 18 moves clockwise with electrode 12 at a positive potential and electrodes 14/16 at a negative potential. In a third mode of operation, electrode 12 is at a negative potential and electrodes 14/16 are at a positive potential with belt 18 moving counter-clockwise. In a fourth mode of operation, electrode 12 is at a negative potential and electrodes 14/16 are at a positive potential with belt 18 moving in a clockwise direction. Generally, for positively-charged product particles, the first operational mode is preferred, while for negatively-charged product particles the third mode of operation is preferred.
Another important feature of the belt-type electrostatic separator is the ability of the belt to sweep the electrodes clean and thus prevent the adherence of layers of material on the electrodes. In this regard, the belt undergoes substantial frictional forces due to contact with the particles, electrodes and oppositely traveling belt segment, and is stretched substantially taut in the longitudinal direction (between the end rollers) during use. This leads to wear of the belt which can adversely affect the quality of the separation over time.
The two effects caused by the belt, transporting material and sweeping the electrodes clean, are both known to be important to the quality of the separation. When the electrodes are uncharged, the geometry of system 10 is generally symmetrical about centerline 25 since belt 18 creates a symmetrical flow field parallel to and within the electrodes. However, when the electrodes are charged with opposite polarity as discussed above, an asymmetry is introduced in this system 10. Furthermore, the charging of the components of the particle mixture creates an asymmetry. It is these two asymmetries that results in the electrostatic separation of components having dissimilar charge.
Typically, it is presumed that symmetrical effects, i.e., those that affect particles irrespective of their electrostatic charge, would not yield asymmetric results, such as improved separation. However, surprisingly it has been found according to the present invention that what may be considered a symmetrical change has produced a significant positive effect on the quality of the separation.